1. Field of the Invention
The present invention is directed to selected chemically modified chelate resins and their use to remove multivalent ions from resist components. In particular, this invention relates to a method for removing multivalent metals (including iron, chromium, copper, cobalt, and calcium) from a resist component solution or resist composition solution by contacting that solution with these selected chemically modified chelate resins.
2. Brief Description of the Related Art
Impurity levels in photoresist compositions are becoming an increasingly important concern. Impurity contamination, especially by metals, of photoresists may cause deterioration of the semiconductor devices made with said photoresists, thus shortening these devices' lives.
Impurity levels in photoresist compositions have been and are currently controlled by (1) choosing materials for photoresist composition which meet strict impurity level specifications and (2) carefully controlling the photoresist formulation and processing parameters to avoid the introduction of impurities into the photoresist composition. As photoresist applications become more advanced, tighter impurity specifications must be made.
In the case of novolak resin materials used for making positive photoresists, such novolak resins have been subjected to distillation or crystallization purification operations in order to remove impurities, especially metals. However, such operations have deficiencies. One, they are time-consuming and costly. More importantly, they do not remove impurities down to the very low levels now needed for advanced applications (i.e., in very low parts per billion maximum levels; typically below 50 parts per billion by weight for each metal).
Ion exchange resins have been used for novolak impurities. One general technique is to pass an impure novolak resin solution through a particulate cation exchange resin (e.g., AMBERLYST styrene sulfonate-divinyl benzene cation exchange resin). However, such treatments have several problems associated with it including the following:
1. The cation exchange resin treatment of the novolak may decrease the pH of the novolak-containing solution, possibly causing serious corrosion of metal containers in which the purified novolak-containing solution may be stored.
2. The purified novolak may have a decreased rate of dissolution during the development step of the photoresist which may be caused by the undesired adsorption of the lower molecular weight portion of novolak resin onto the cation exchange resin.
3. Alkali metals such as sodium and potassium are easily removed with conventional particulate cation exchange resins. However, divalent or trivalent metal cations (e.g., Cu.sup.+2, Ni.sup.+2, Zn.sup.+2, Fe.sup.+2, Fe.sup.+3, Ca.sup.+2, or Cr.sup.+3 ions) may have a lower affinity to conventional cation exchange resins. Iron and other easily oxidizable metals cannot be completely removed because they may be colloidal metal hydroxides or oxides. Such colloidals are not significantly removed by cation exchange resin treatment.
4. Ion exchange resins, particularly strong acid-type of cation exchange resins, decompose resist components which contain or use solvents containing hydrolyzable groups such as esters. For example, ethyl lactate is decomposed by AMBERLYST A-15 to form polylactide moieties, which may degrade lithography performance of photoresists. As used herein, that term "polylactide" is defined as a polymeric or oligomeric product of a lactide, a cyclic dimer of lactic acid which is formed by hydrolysis of ethyl lactate.
In addition to the standard cation exchange resin treatment of the novolak resin, it is known to subject complete photoresist compositions (e.g., novolak resin, photosensitizer, and solvent) to both cation and anion exchange resin treatment. For example, Japanese Patent Publication (Kokai) No. 57-74370 discloses a method of reducing impurities in silica paint film liquid coating by suing cation exchange resins and anion exchange resins in separate and a successive manner. Japanese Patent Publication (Kokai) No. 01-228,560, which was published on Sep. 12, 1989, teaches that the metal impurities content in photosensitive resin solutions or photoresist compositions may be reduced with a mixture of a cation and anion exchange resins. However, these techniques have the deficiency of not removing divalent and trivalent metal impurities and may decompose resist components or solvents containing resist components. Usually, such cation and anion exchange resins have been washed with a solvent such as deionized water or the same solvent in which the resist component is already dissolved in. However, such washings with water or solvents will not clean the resins of preattached metal impurities because metal ions such as sodium or potassium as well as other acidic contaminants strongly bind to the anionically charged groups of cation exchange resins.
There are several ways now known to remove multivalent ion metal impurities such as iron, chromium, copper, calcium, and the like from resist components. These include (a) microfiltration, (b) clay treatment, and (c) conventional chelate resin treatment. All three methods are successful in removing large amounts of such multivalent impurities with many types of resist components; however, each of these methods may be either very slow or not always effective when attempting to obtain certain resist components having multivalent metal impurity levels below 50 ppb.
U.S. patent application Ser. No. 07/753,488, which was filed on Sep. 3, 1991 with Kenji Honda, Edward A. Fitzgerald, and Lawrence Ferriera as the co-inventors, is directed to a method for removing metal impurities from a resist component solution or a resist composition solution by contacting that solution with a cation exchange resin and a chelate resin. One of the chelate resins disclosed in this application is DIANION CR-20 polyamine-type chelate resin produced by Mitsubishi Kasei of Tokyo, Japan. This type of chelate resin advantageously will not decrease the pH of a photoresist solution or novolak solution during a metal ion removal step. However, this chelate resin, as well as other known chelate resins, will not consistently remove substantially all of the polyvalent metal ions from the solution (e.g., less than 100 ppb remaining for each polyvalent metal). Stronger chelating resins are needed for this purpose without lowering the pH of the solution.
Accordingly, there is still a need in the photoresist art for improved methods of removing metal impurities from novolak resins and other materials used as photoresist components without otherwise adversely affecting other properties of the resist component. The present invention is a solution to that need.
Separately, U.S. Pat. No. 4,446,284, which issued to parker on May 1, 1984, is directed to a process for producing polymers that contain pendant unsaturation. In particular, that process involves reacting a polymer containing a pendant phosphonium salt with an aldehyde in the presence of an alkali ionizable functionality to yield a polymer having a pendent unsaturation. See formula III in column 3. Among the polymer precursors listed as being suitable for this reaction is styrene/divinyl benzene. Among the suitable aldehyde reactants is 3,4-dihydroxybenzaldehyde. It is also noted that the resulting compounds may be used as chelates (see U.S. Pat. No. 4,446,284). However, this reference does not teach or suggest using polymer precursors with amine functionalities. Also, the reference does not teach or suggest that its reaction products may be used to remove multivalent metal impurities from any material, let alone resist components, as presently claimed.